1. Field of the Invention
The present invention relates to a photodetector and more particularly to a photodiode current reading circuit that enables the dark current of the photodetector to be eliminated to the utmost possible extent.
Photodetectors such as those used for example in visible or infrared imaging are formed by two main elements: a photosensitive element and a multiplexer. The photosensitive element is formed by photodiodes that are arranged in linear arrays or matrices and enable the generation of charges by photoelectrical effect. The multiplexer converts and transmits the charges thus generated by producing a useful electrical signal.
2. Description of the Prior Art
According to the prior art, each photodiode is reverse-biased at a relatively high voltage. It follows therefrom that the reverse current that flows through the diode when there is no radiation, commonly called a dark current, is itself also relatively high. When there is a low-amplitude radiation, the detected signal then contains a high noise level. This is of course a drawback.
Another drawback lies in the fact that the different photodiodes forming the array or the matrix are not perfectly identical.
It follows therefrom that their dark currents have different values. It is then necessary to perform an operation to calibrate the detector so as to take account of this variation. This calibration is a painstaking operation.
The invention does not have these drawbacks.
An object of the present invention is a photodetector comprising means enabling the elimination of the dark current and hence of the detection noise associated with this dark current.
According to the invention, instead of supplying the photodiode with a negative bias voltage, a non-linear load acting as a voltage limiter is connected to the terminals of the photodiode. This non-linear load tends to keep the bias of the diode close to zero volts. A current mirror type of copying circuit is used to copy the current in the non-linear load and the current thus copied is measured. The photodiode is not supplied by a voltage source as in the prior art. The bias voltage that is set up at its terminals is due solely to the photoelectrical energy generated by the photodiode.
Through this assembly, the dark current is low, for the photodiode remains always biased at a very low voltage for which the dark current is minimal. This current remains low both at the start of an illumination measurement stage and at the end of this stage. In the prior art, the initial bias of the photodiode would prompt a high dark current and, furthermore, the value of the dark current would vary between the start of the measuring phase and the end of the measuring phase. The variation would then be a function of the illumination. The average dark current during the measurement was all the greater as the measured illumination was low. This was particularly harmful. These drawbacks have been eliminated by the invention.